This nucleus wraps around the thalamus, forming a thin net, for which it is named (reticular means netlike or an intricate network). The thalamic reticular nucleus, when active, inhibits the sensory nucleus with GABA.

The sensory nucleus is positively stimulated by acetylcholine, while the reticular nucleus is inhibited by acetylcholine.

This means that when the ascending reticular activation system is active, as during waking hours, the inhibitory actions of the reticular nucleus are inhibited.

The thalamus is never fully stimulated to an action potential via these projections from the ARAS, but it is sensitized by them.[1]

During sleep, when the ARAS shuts down, the reticular nucleus is free to inhibit the sensory nucleus of the thalamus. This is how the reticular formation mediates attention and wakefulness.

This makes sense because during wakefulness, it is easy to take in sensory stimuli.

Once the ARAS system begins to shut down, the world seems duller, and it is much harder to take in information from the outside world.

The chemical equivalent of this would be if the thalamus was being partially inhibited by GABA, making it more difficult to relay information to the cortex.